Weird Science

Running makes you lean and strong. It also gives you twitchy legs, black toenails, and an urgent need to find a bathroom now. Why? Here, doctors, therapists, and physiologists offer explanations and practical solutions for our most perplexing body issues.

If your rubbery, burned-out gams had a fuel gauge, it would be firmly on "E." For beginners, the needle may arrive there as a result of sheer effort. "If your muscles aren't familiar with a new movement, they become inefficient at contracting and can't work in a coordinated manner, which results in shaking," says Michele Olson, Ph.D., C.S.C.S., professor of exercise science at Auburn University in Montgomery, Alabama. (Veteran runners might experience this phenomenon when they attempt push-ups on feeble arms.) For others, it could be that you started too quickly. "When you go out too hard, the oxidative system doesn't kick in as smoothly as it does when you warm up and work up to a pace," McDaniel says. "It's like shifting gears too quickly in a car. You deplete your energy levels prematurely." The other cause is simply that your muscles are depleted of electrolytes and glycogen—easily accessible fuel on which they run—and the shaking is their way of telling you to fill 'em up.

Running Rx Warming up prerun is key for beginners and vets. Start slow, and ease into your ultimate goal pace. If you're running hard for more than 45 minutes, drink eight ounces of sports drink about 20 minutes before you run; the carbs will keep your muscles humming. Postrun, if you're trying to shake the shakes, walk around, stretch gently, and grab quick fuel, like a sports drink.

Why does coffee speed up more than just my legs?

A pre-run prereq for many runners to clear the system on their own terms, java stimulates the muscles in the GI tract faster than Mother Nature; some reports say coffee jolts your system in as little as four minutes. Once you're out on the road, proceed with caution: Many energy gels have caffeine in them, which may cause your intestines to move as quickly as your legs.

Running Rx In the weeks before an important run or race, determine how much coffee you need for an evac, then sip and lighten your load accordingly. Also, figure out if you can tolerate caffeinated gels. Plan B: Pick a route with a few public restrooms along the way, so you can properly do your business.

Why do I feel nauseated after a long run?

You put in 18 miles to be able to eat a burrito, not to feel pukey thinking about one. Blame the decreased appetite on chemistry; a 2008 study published in the American Journal of Physiology found that a 60-minute session of treadmill running increased the amount of the gut hormone peptide YY, an appetite suppressant, and suppressed acylated ghrelin, an appetite stimulant. Full-on nausea? "There's a good probability you haven't fueled properly during the run," says Ilana Katz, M.S., R.D., C.S.S.D., a sports nutritionist in Atlanta. A lack of fuel in your body sends it into a stressed mode, that fight-or-flight mentality where survival—not eating rice, beans, and guac—is key.

Running Rx Try to prevent the problem by taking in about 60 grams of carbs per hour, either through a sports drink, gel, or regular food during your run. "The body can process about one gram of carbs per minute," says Katz. Postrun, try to knock back something easy, like a recovery drink, within 30 minutes. If you can't eat right away, don't worry too much. "Appetite loss is typically short-lived," says Katz. "Within an hour or two, suddenly you'll have a major one."

Why do I get headaches during or after a run?

It's not just because you know you're returning to the mess you ran away from. Headaches stem from a range of causes, from simple (a too-tight hat) to complex (a proclivity for migraines). Two of the most common reasons are tight muscles and poor hydration. "The trapezius attaches high on your scalp, so if you hold a lot of tension in your upper body as you run, your head could ache," says Dr. Bright. Headaches are also a symptom of both underdrinking and overdrinking.

Running Rx Shake out your arms and hands and teeter-totter your neck as you run. At home, hold your left ear toward your left shoulder, right toward your right; repeat with the chin. Nail your beverage needs by weighing yourself before and after an hour run (without drinking). Each pound lost equals 16 ounces of fluid you should drink per hour.

Why do my bending knees sound like Rice Krispies when I walk down the stairs?

Snap, crackle, pop? Crepitus, the medical term, happens when cartilage, the connective tissue between bones, starts to age, says James Wyrick, M.D., orthopedic surgeon and associate professor at the University of Cincinnati in Ohio. We all start life with quiet, smooth cartilage between our kneecaps and thigh bones, but over time, it becomes gray and old and doesn't regenerate; most people older than age 30 have some mild crepitus. Weak quads or a tight IT band can pull the kneecaps out of alignment and exacerbate the wear and tear.

Your knees pipe up when they bend past 30 degrees because the kneecap tracks into a groove in your femur—that is, cartilage-weak bone grinds into cartilage-weak bone. "The intensity of the pressure and the different contact points in the groove make the noise," says Dicharry.

Running Rx "Cracking knees may lead to problems down the line, like arthritis," says Dicharry. Minimize that chance by strengthening the muscles that control the hips and knees, and keep your lower half in alignment, such as clamshells for the hip; squats for the knees.

Why is it easier for me to run in the morning and so hard to rally at the end of the day—or vice versa?

Your natural bird persona—lark or owl—is partly determined by genetics. Housed in the hypothalamus, the portion of the brain that also controls sex drive and appetite, your biological clock is difficult to alter. If your forebears coherently discussed the Middle East situation at 7 a.m., you're likely to feel sharp before the sun comes up, too. If they thought 9 p.m. was the perfect time for dinner, you probably are happy staying up late. "Natural morning people seem to hit their lowest body temperature earlier in the night than evening people do," says Chris Kline, an exercise physiology researcher at the University of South Carolina who specializes in sleep research. "Their body temperature is warmer when they wake up, so they're much more ready to go."